I have an 80cc Chinese two stroke engine on my bicycle and wish to experiment with changing the spark timing. Apparently the magneto gives off an alternating voltage of around 100 volts which is used both to power the CDI unit and as the spark trigger signal. The alternating voltage (and trigger signal) frequency is directly associated with a single rotation of the motor and rotating magneto magnet which is attached directly to shaft of the piston crank.

It would appear that the alternating signal is stored in a capacitor in the CDI unit until the spark trigger signal is processed (once per rotation), whereupon the charge in the capacitor is release into the ignition coil causing the spark.

If anyone can give me a circuit to adjust the spark trigger signal timing I will be most grateful.

You could delay (retard) the signal electronically, but not advance it.
To advance it you will have to move either the magnet or the trigger coil.

Click to expand...

Agree.
Out of the box idea? Run a PLL from the trigger signal so you can predict when the next trigger will be then you can issue an earlier trigger. It would need some careful precautions to avoid generating a spark at a stupid time. If you don't value the engine highly it might work.

Assuming the positive signal in the alternating voltage is the trigger to discharge the capacitor and create the spark, if we changed the phase of the incoming signal to the CDI, surely this would change when the spark is produced?

Has anyone got a circuit which can adjust the phase of a fairly hefty incoming (from the magneto) alternating signal, perhaps 100volts and an amp or two before it enters the CDI?

Original question:
I have an 80cc Chinese two stroke engine on my bicycle and wish to experiment with changing the spark timing. Apparently the magneto gives off an alternating voltage of around 100 volts which is used both to power the CDI unit and as the spark trigger signal. The alternating voltage (and trigger signal) frequency is directly associated with a single rotation of the motor and rotating magneto magnet which is attached directly to shaft of the piston crank.

It would appear that the alternating signal is stored in a capacitor in the CDI unit until the spark trigger signal is processed (once per rotation), whereupon the charge in the capacitor is release into the ignition coil causing the spark.

If anyone can give me a circuit to adjust the spark trigger signal timing I will be most grateful.

It can be done electronically but to do so reliably requires considerably more electronics than most people would care to try and design and build in order to make it work. Especially so for a odd or old engine that does not support anything but the bare basics for an ignition system like yours.

From personal experience and experimenting done years ago in college the most effective 2 cycle motorcycle ignition design I could come up with was a simple high voltage flyback transformer (from an old CRT monitor or TV I junked out) based design that was set up so that whenever the points were open the flyback transformer primary side driver circuit (basic two transistor push-pull oscillator) would fire up at whatever resonance frequency it wanted to run at (25- 35 KHZ more or less) and stay on until they closed giving the system a hot spark with very long dwell time that did wonders for an old two cycle engines operation.

It worked surprisingly well for it's simplicity! Good starting, idle and running power from old junk parts.

Then one week while I was at school my brother and his friends ran it on straight gas and destroyed the engine beyond reasonable repair.

These cheap Chinese engines are absolutely solid with their stator and rotor mounting. No adjustment at all. The rotor is fixed to the shaft by a pin and the stator with four bolts to the body of the motor.

I have an 80cc Chinese two stroke engine on my bicycle and wish to experiment with changing the spark timing. Apparently the magneto gives off an alternating voltage of around 100 volts which is used both to power the CDI unit and as the spark trigger signal. The alternating voltage (and trigger signal) frequency is directly associated with a single rotation of the motor and rotating magneto magnet which is attached directly to shaft of the piston crank.

It would appear that the alternating signal is stored in a capacitor in the CDI unit until the spark trigger signal is processed (once per rotation), whereupon the charge in the capacitor is release into the ignition coil causing the spark.

If anyone can give me a circuit to adjust the spark trigger signal timing I will be most grateful.

Click to expand...

They always used to have a stator plate that you could slacken 3 screws and rotate it a small amount.

With most ignition sensors of this type, automatic advance is inherent in the design - as the rotor "pip" approaches the trigger coil faster; the voltage risetime increases.

Here's an interesting thought... using a transformer to change the phase 180 degrees... I'm sure by including other elements in the circuit we could make the phase variable, and thus vary the spark timing through the CDI... Voila.

Has anyone got more thoughts on this and/or can add some specific electronic detail to enable variable phase shift and thus spark timing?

I believe the alternating drive voltage from the magneto stator is around 100 v a/c.

I wonder if there's any relationship between phase change through a transformer and frequency (ie. speed of the engine and hence the magneto)? All thoughts gratefully appreciated.